This application is based on applications No. 13754/1997, 13755/1997 and 35430/1997 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a development apparatus and a development method utilized for developing an electrostatic latent image formed on an image bearing member in an image forming apparatus such as copying machines, printers and the like, and more particularly to a development apparatus and a development method adapted to produce an image featuring an adequate image density and good gradations including halftone and to prevent toner particles of a developer material from being scattered out of the apparatus or carrier particles of the developer material from being adhered to the image bearing member even when high-speed image forming operations are performed.
2. Description of the Related Art
Various types of development apparatuses have conventionally been employed by the image forming apparatuses such as copying machines, printers and the like, for developing the electrostatic latent image formed on the image bearing member. FIG. 1 shows one type of such development apparatuses.
As seen in the figure, the development apparatus comprises a cylindrical developer carrier member 11 opposing an image bearing member 1 and having a magnet roller 16 mounted to its inner periphery. The development apparatus body 10 contains a developer material 2 therein and includes a developer supply member 12 such as of a bucket roller for supplying the developer material 2 to the developer carrier member 11.
The developer supply member 12 within the apparatus body 10 is rotated for supplying the developer material 2 to a surface of the rotated developer carrier member 11. The developer material 2 is magnetically held to the surface of the developer carrier member 11 by means of the magnet roller 16, thus transported as the developer carrier member 11 rotates. After a regulating member 13 has regulated the amount of developer material 2 to be transported to a development region between the developer carrier member 11 and the image bearing member 1 in opposed relation, the developer material 2 is supplied to the development region for developing the electrostatic latent image formed on the image bearing member 1.
The development apparatus of the above construction is designed to achieve efficient development of the electrostatic latent image recorded on the image bearing member 1 and to prevent a magnetic brush of the developer material 2 from scraping off a toner powder image formed on the image bearing member 1. That is, as seen in FIG. 1, the developer carrier member 11 is applied with a DC voltage from a DC voltage source 14 and an AC voltage from an AC voltage source 15 in superimposition so that an electric field with an AC electric field superimposed on a DC electric field is applied to the development region between the developer carrier member 11 and the image bearing member 1 in opposed relation thereby performing the development process.
However, where the electric field with the AC electric field superimposed on the DC electric field is applied to the development region between the developer carrier member 11 and the image bearing member 1 in opposed relation thereby developing a halftone image, variations in the surface potential of the image bearing member 1 entail significant variations in image density and hence, a favorable halftone image cannot be obtained. The resultant halftone image does not feature good gradations nor necessarily have a good texture.
In this connection, U.S. Pat. No. 4,610,531 discloses a development apparatus arranged such that a first period (active period) to apply the AC electric field to a gap between the developer carrier member and the image bearing member and a second period (inactive period) to apply thereto no AC electric field are repeated in an alternating sequence while the electric field is invariably applied in a direction to attract the developer material back to the developer carrier member at the end of the first period.
In this instance, as disclosed in the above patent publication, wherein the AC electric field is applied in the first period at the end of which the electric field is invariably applied in the direction to attract the developer material back to the developer carrier member, the developer material moving between the image bearing member and the developer carrier member is invariably attracted from the image bearing member back to the developer carrier member when the AC electric field becomes inactive. In this state, the second period starts while the developer material continues to be attracted back to the developer carrier member.
This leads to a failure to supply a sufficient amount of developer material to the image bearing member in the subsequent period to apply the AC electric field for developing a latent image and hence, a resultant image suffers a reduced image density. Particularly, in the high-speed image forming operations currently practiced, the resultant image suffers an even lower image density as well as poor texture.
Where the development process is performed by applying the AC electric field besides the DC electric field to the development region between the developer carrier member 11 and the image bearing member 1 in opposed relation, as described above, a toner contained in the developer material 2 forms a toner powder cloud which increases the efficiency of the development process.
Unfortunately, such a toner forming the toner powder cloud is apt to be scattered out of the development apparatus body 10 and therefore, the image forming apparatus, such as copying machines, employing such a development apparatus suffers contamination of the interior thereof with the toner thus scattered or produces unwanted fogged images. Additionally, a carrier of the developer material 2 is apt to adhere to the image bearing member 1 because of the effect of the aforesaid AC electric field, so that a resultant image suffers carrier fogs.
In recent attempts to overcome such drawbacks, various constructions have been disclosed. For example, Japanese Unexamined Patent Publication No. 60(1985)-125863 discloses a development apparatus arranged such that an electrode member is disposed downstream of the developer carrier member in a direction of the movement of the image bearing member as opposed by the image bearing member, the electrode member applied with a similar combination of AC and DC voltages to that applied to the developer carrier member so that toner particles scattered out of the apparatus body may be reduced. Further, Japanese Unexamined Patent Publication No. 59(1984)-223467 discloses a development apparatus arranged such that an electrode member is disposed in the development region between the developer carrier member and the image bearing member in opposed relation as interposed therebetween and an AC voltage is applied to at least one of the electrode member and the developer carrier member thereby performing the development process under the AC electric field.
However, in the instance of Japanese Unexamined Patent Publication No. 60(1985)-125863 wherein the electrode member is applied with the same AC electric field as that applied to the developer carrier member, when the toner forming the toner powder cloud is moved into the gap between the electrode member and the image bearing member in the development region defined between the developer carrier member and the image bearing member in opposed relation, the AC electric field applied to the gap between the electrode member and the image bearing member causes the toner in the form of the toner powder cloud to be scattered out of the development apparatus body. Thus, a failure of adequately controlling toner scattering results.
In the instance of Japanese Unexamined Patent Publication No. 59(1984)-223467 wherein the AC voltages are applied to the developer carrier member and the electrode member, respectively, in a manner such that the AC voltages of the same frequency are applied to the developer carrier member and the electrode member in synchronism, a potential difference between the developer carrier member and the electrode member stays constant so that the gap between the developer carrier member and the electrode member is not subject to the AC electric field. This results in a failure of allowing for an adequate dispersion of the toner particles in the developer material. If, on the other hand, AC voltages of different frequencies are applied to the developer carrier member and the electrode member, a potential difference between the developer carrier member and the electrode member temporarily increases to an excessive level. This results in the occurrence of current leakage between the developer carrier member and the electrode member.